I wouldn't be the least surprised by a 24-70mm f/2.8 IS USM. It might have lower IQ than the IS-less' lens, as I think some people would be willing to compromise on price & IQ to get IS for the hand-hold-ability.
As for another 24-70mm, I'm not so sure. If it's f/4 IS-less, how would it compete with the cheap 24-105mm f/4 IS USM? What else could it be?
These comments surprise me, if only because the Tamron 24-70 VC offers exactly what you say people are looking for at a price that already seriously undercuts the Canon non IS version. I don't personally think that a new version is worth more than the original price point of the 24-70LII unless it offers something more than just IS, particularly if that comes at the cost of higher weight and slightly compromised optics.
Exactly - Tamron is taking sales away from Canon, so I expect Canon to release a lens to compete with it.